The present invention relates to a method for producing a magnetic recording medium and particularly relates to a method for producing a magnetic recording medium adapted to high-density recording and having a magnetic layer in which magnetic particles have a vertical component at an arbitrary angle with respect to the direction of a support.
Heretofore, magnetic recording media such as magnetic tape, floppy disks and the like are generally produced utilizing the steps of applying a coating composition, obtained by dispersing ferromagnetic fine powder into a binder dissolved by an organic solvent, onto a belt-shaped non-magnetic support while continuously moving the support longitudinally; solidifying the coating composition by drying; and cutting or punching the resulting product into the desired size.
Particularly in recent years, the demand for increasing the information recording density per unit area of magnetic recording media has significantly increased in order to increase storage capacity of the magnetic recording media.
To increase the recording density, the write flux generated by a magnetic head must be concentrated on a small area, and the size of the magnetic head must be correspondingly reduced. With the reduction in size of the magnetic head, the quantity of flux generated is generally unavoidably reduced. Hence, the volume of the magnetic recording layer in which the direction of magnetization can be inverted by the reduced magnetic flux is necessarily smaller. If the volume of the recordingly layer is not reduced, perfect inversion of magnetization cannot be provided.
For this reason, in order increase the recording density per unit area it is necessary to reduce the thickness of the magnetic recording layer.
Other recording methods have been proposed for increasing the recording density including, for example, orienting the axis of magnetization of the magnetic layer in the direction of thickness of the magnetic medium (perpendicularly) and providing a vertical component of the axis of magnetization at a specific angle to the direction of the thickness of the recording medium. However, these methods have met with little success because the magnetic particles oriented in a predetermined manner tend to move to a position where they are arranged in the same direction as the plane of the magnetic recording medium, as in a conventional arrangement. That is, the magnetic particle tend to move after the coating process and before the magnetic particles are perfectly dried. This suggests that magnetic particles in the not-dried magnetic layer are unstable so that their orientation may be effected by factors, such as vibration associated with movement of the support, air pressure against the surface of the magnetic layer as a result of the drying process and a liquid flow phenomenon which is produced in the magnetic layer when the solvent in the magnetic layer is vaporized.
To prevent the magnetic particles from being reoriented, efforts have been made to increase the viscosity of the magnetic coating composition. In particular, Japanese Patent Publication No. Hei-3-38653 discloses a method wherein the viscosity of the magnetic coating composition was increased by preparatory drying. The biggest disadvantage associated with this method is that a relatively large amount of energy is required to maintain the orientation to any measurable degree.
As noted above, there have been proposed many methods and apparatuses in which this type orientation is completed by drying while applying a perpendicular or oblique magnetic field. An example of a convention method of this type is shown in FIG. 7.
A magnetic recording medium producing apparatus 10 shown in FIG. 7 performs a series of producing steps including an initial step of unwinding a support 3 from a delivery roll 2 and a final step of rewinding the support 3 on a take-up roll 9. The support 3 unwound from the delivery roll 2 is multiply coated with predetermined coating compositions by a multilayer coater 4. For example, the lower-layer coating composition ejected from a forward slit 5 may be an undercoat non-magnetic composition or may serve as a magnetic layer if an undercoat layer has been already applied. On the other hand, the upper-layer coating composition ejected from a rearward slit 6 may be a magnetic coating composition (in the case where the lower layer is formed from the magnetic coating composition, the upper-layer coating composition is another magnetic coating composition). After the coating step, the resulting medium is dried by a dryer 8 while a magnetic field oriented at a predetermined angle is generated by opposing magnets 7a and 7b. Thereafter, the support is rewound onto the take-up roll 9.
Thus, even in the case where the conventionally proposed method is used, the aforementioned reason makes it difficult to orient the magnetic particles in a desired direction and to fix the magnetic particles without adversely effecting the orientation of the particles.
Based on the foregoing, an object of the present invention is to provide a magnetic recording medium which is optimum for high-density recording where the magnetic particles are oriented in the direction of the thickness of the magnetic layer, as described above, and are maintained in that position. Another object of the invention is to provide a method for producing the same.